Method of brightening metals electronegative to iron



Patented Jan. 9, `1940 UNITED LSTATES METHOD 0F BRIGHTENING- METALS ELECTRONEGATIVE T0 IRON n George Dubpernell, Waterbury, Conn., and Karl Gustaf Soderberg, Detroit, Mich., assignors, by mesne assignments, to The Udylite Corporation, a -corporation of Delaware Application October, 26, 1936, Serial No. 107,604

In France June 28, 1933 7 Claims.

Y This invention relates to a method of treating metals electronegative to iron and belonging to the second sub-group of the second groupl of the periodic system as given on pages 522-3 in the 16th edition of Handbook of Chemistry and Physics by Hodgman and Lange, to improve the surface-appearance and produce a high lustre, and in particular to a method of treating a coating of a metal electronegative to iron such as cadmium and zinc to give the coating a high lustre. This application is a continuation in part of application Serial No. 45,416, filed October 17, 1935, which is a division of application Serial No. 627,422, filed August 4, 1932 and now matured into Patent No. 2,021,592.

In the plating of these metals which are elec-A tronegative to iron, in particular such-as cadmium and zinc, it is common practice to use addition agents in the plating solution which tend' to produce deposits which are bright and lustrous.

The lustre which is sometimes imparted to cad.

mium plate is very similar to that obtained by bung. In the case of articles which have an irregular surface contour the metal coating in the recesses does not have the same high lustre as that on the protruding parts. Very often the electrodeposited coatings in these recesses are. dull owing to too high or too low a concentration of the addition agent or that the addition agent is insulciently active. Hence, it has been proposed to dip the article subsequent to the plating operation in a brightening dip. Among the various dips which have heretofore been proposed are solutions of non-oxidizing acids such as acetic, oxalic, cyanic and weak solutions of sulphuric and hydrochloric acids, but a solution of any one of these acids only has a whitening effect on metals electronegative to iron such as cadmium and zinc. This whitening effect is probably due to removal of a thin lm of oxide from the surface and is distinct from, and should not be confused with, the brightening or obtaining of a high lustre on the surface of cadmium or zinc coatings. It has also been proposed to use dilute nitric acid as a dlp for producing a bright or a highly lustrous cadmium coating, but this dilute nitric acid solution has the disadvantage of accelerating or causing tarnishing of the cadmium surface.

It is an object of this invention to produce a method and dip for brightening metals which are electronegative to iron and in particular such metals as cadmium, zinc and magnesium without causing the metals to tarnish. y

It is an object of this invention to produce a method and dip for brightening electrodeposited coatings of metals electronegative to iron which have been electroplated from a cyanide bath, such as cadmium and zinc.

This invention also contemplates a method of treating metals electronegative to iron by immersion in a dip containing chromic acid (Cros) and an active acid radical of the nature of a catalyst and also the production of this type of brightening dip.

We have discovered that an aqueous solution of chromic acid, to which has been added a proportion of certain active acid radicals, gives a very satisfactory lustre to dull surfaces of cadmium and zinc which may be electrodeposited, sprayed, rolled, cast, etc., as Well as to rolled magnesium 15 strips. These active acid radicals include the sulphate, the chloride and the nitrate radicals.

Pure chromic acid solutions do not brighten the plate. This fact is undoubtedly related to their inability to dissolve cadmium, zinc and magnesium. The addition of active acid radicals causes an attack on the metal. These active acid radicals act in the nature of catalysts. Inactive radicals. do not generally cause solution of the plates. The pure chromic acid with or Without the addition of inactive radicals may have a slight whitening eifect similar to that of a number of Weak acid solutions, but this effect should not be confused with the brightening effect which is obtained by means of our invention. It appears as if certain crystal faces in the surface are attacked preferentially to others in our solutiton, thereby greatly increasing the reflectivity of the surface and producing a brilliancy similar to that of a buffed surface. 5

The accompanying graph, Figure l, which is practically self-explanatory shows that the rate of solution of the cadmium plate increases with increasing chromic acid and acid radical contents. This gure also shows that the desired time in '40 seconds of immersion in the brightening solution of the metal which is to be brightened, decreases with the increasing rate of Isolution of the plate.

In other Words, excellent brightness is associated with a relatively high rate of solution. 45

- Referring to the graph shown in Figure 2, it will be seen that the ratio of the chromic acid to the acid radical content is important. When this ratio in grams per liter of chromic acid (CrOa) to grams per liter of sulphate radical (S04) falls below about 20 (i. e. when the grams per liter of chromic acid is less than twenty times the grams per liter of sulphate radical) a brown lm consisting probably of chromium chromate which is insoluble in water is formed on the surface of the cadmium coating. This film can be removed by rinsing the surface of the cadmium coating in any of the solutions described below. When the ratio of chromic acid in grams per liter to sulphate radicals in grams per liter is above about 2G, no insoluble film is formed and the brightened cadmium metal can be rinsed in water and the acid rinse is unnecessary. This critical ratio at which the brown film forms on the cadmium is not absolutely 20 but ranges above and below 20 a :iew points and in general between about 18 and 32.

Figure 2 shows the relation between the appearance of the plate and the composition the solution. 'Ihis chart is based on experiments at ordinary room temperature. 'I'he effect oi increasing the temperature is not particularly great except that it moves the area. within which bright plates are obtained to include higher ratios.

In general, a distinct brlghtening effect can be obtained with from 25 grams per liter chromic acid up to the saturation point when the ratio falls between 1 and 400. At the extremities of the ratio range, higher chromic acid content is desirable, such as 100 grams per liter at a ratio of both 1 and 400.

The most effective ratio falls between 1 and 60, with a desirable chromic acid content from 125 grams per liter and up at a ratio of 1 and 50 grams per liter and up at a ratio of 60.

As the rate of solubility of the cadmium plate increases with the chromic acid content and the thickness of the plate is originally rather low (in the order of .0002-.0010 in.), we prefer to use a moderate chromic acid content. This is all the more advisable from an economic standpoint since the solution clings to the plate and considerable quantities may be lost as drag-out in the subsequent rinsing operation. Qur preferred solution for work plated in still tanks and automatics, therefore, consists of 150 grams per liter chromic acid (CrOa) plus 3.75 grams per liter sulphuric acid (HzSOi) which is equal to a ratio of about 40. A to l0 second immersion in this solution is sufficient to bring forth the desired lustre. No acid rinse is necessary following the immersion in this bright dip.

Work plated in bulk in barrels cannot be handled as fast as other work and therefore the time of attack on the coating is longer. We, therefore, prefer to use a weaker dip for barrel plated work, containing 100 grams per liter chromic acid (CrOa) and 2 grams per liter sulphate radical (S04) giving a ratio of 50.

Chloride (Cl) or Nitrate (NO3) radicals can be substituted for the sulphate radical without materially changing the conditions. These radicals are perhaps slightly less active and the ranges of concentrations not quite as wide. The critical ratio of chromic acid to active acid radical, below which the rinses described below must be employed to remove-the brown film from a cadmium coating, falls at about l0 for Cl and at about 12 for NO3, when grams per liter is chosen for the unit.

The general description of the action of our new dips on cadmium plate is also applicable to their action on zinc plate. The lustre obtainable on zincplate is not quite as brilliant as that on cadmium plate. 'I'his is probably a consequence of the dierence in physical properties of the metals themselves. It is well known that bufl'ed cadmium plate has a higher lustre than buffed zinc plate.

The ratio of chromic acid to sulphate radical concentrations, above which the dip is free rinssancerre ing and no acid rinse is needed, is practically the same for zinc as for cadmium plate. When the chloride radical is used, this limiting ratio has such a high value, above that of the best ratios for obtaining a high lustre on the zinc coating that a rinse in one of the below described solutions appears necessary to remove the brown lm.

We have found that a mixture of active acid radicals can be used just as well as single active acid radicals. The eiect of the various radicals appears to be additive.

When cadmium, zinc and the other metals of this group'are dipped in a chromic acid-sulphuric acid bright dip having a ratio in grams per liter of Cros/S04 of less than about 20 or in a chromic acid-hydrochloric acid bright dip having a ratio in grams per liter of CrOs/Cl of less than about or in a chromic acid-nitric acid bright dip having a ratio in grams per liter of Cros/NO3 of less than about 12, a thin film is left which covers the brightened surface of the metal. This nlm is usually brown and plainly visible in the case of cadmium and usually greenish and somewhat less visible in light in the case of zinc. When this thin lm is properly removed, there is left a bright surface. 'I'his film can be removed by dipping in, or otherwise subjecting it to, solutions of acids and alkalles. This film can be removed by dipping the same at room temperature for a period of time ranging from a second up to fifteen minutes in a solution of any acid having a pH value of 2.68 or less. Solutions of the following acids having a pH value of 2.68 or less, effectively remove this film: sulphuric acid (H2804), hydrochloric acid (HC1), phosphoric acid (H sPOi), acetic acid (CHBCOOH), hydrofluoric acid (HF), citric acid (CeHaOv), formic acid (HCOOH), hydroiluosilicic acid (HzSiFe).

There is set forth below some of the results of numerous tests showing both the approximate minimum concentration of each acid which will remove the film and the approximate maximum concentration which does not remove the film. The solution concentrations are given in per cent by volume of the concentrated liquids and g./1 of solids. All materials were C. P. grade unless otherwise stated.

onceitnlajoncerkixtira]- on w c on w c Acid removes pH does not re- DH film move lm H5504 (Sp. gr. 1. 4) 005% 2. 67 000570 3. 68 HC] (sp. gl'. 1.19) .05% 2. 23 .01% 2.9

:P04 (Sp. gf'. 0.1% 2.12 01% 2. 85 CH; COOH (glacial) 1% 2. 57 0. 1% 3. 0S (52%) 05% 2. 43 01% 2.85 Citrlc (crystals) l0g.l1. 2. 15 lgJl. 2. 7 HCOOH (sp. gr. 1.20). 0.5% 2. 26 0.1% 2. 72 HzSiFt (sp. g1'. 1.27) .1% 01% Highest pH which removes film 2.67. Lowest pH which does not remove film 2.70.

Thus it has been found that the critical pH value above which the film will not be removed falls at approximately 2.68. However, these acids are mentioned for descriptive purposes only and vnot by way of limitation because a solution of any acid having a. pH value of approximately 2.68 or less, will remove this discoloring film. It was found that where the pH value of the acid solution was more than approximately 2.68, for example, 2.70 or 3.68, that is, solutions of lesser acidity or lower hydrogen ion concentration, that thislm was not removed. 'I'he criti- Substance Concentration Time Remarks Gram per liter Minutes N aOH- 0.1-150 15 Film not entirely removed.

Do 175-535 l5 Film removed. Zinc visibly attacked. KOH 19d-222 15 Film not removed.

Do 15 Film removed.

These values are the same for both zinc and cadmium.

The normality of a solution of 150 grams of sodium hydroxide per liter of solution is 3.75; that of 175 grams of sodium hydroxide per liter of solution is 4.37; that of 222 grams of potassium hydroxide per liter of solution is 4.00; and that of 253 grams of potassium hydroxide per liter of solution is 4.5 normal.

Thus it has also been found that the critical normality below which the caustic or alkali solutions will not remove the film falls at approximately 4.25.

'I'hese caustic solutions were operated at room temperature and the film was subjected to the action of the caustic solution for a period of time not exceeding 15 minutes. The time required for removing the lm varies generally inversely with the concentration of the solution, that is, the greater the concentration of the potassium or sodium hydroxide solution, the less the time required for removing the film. In the acid solution the time required varies directly with the pH value of the acid solution, that is, the higher the pH value the longer the time required. I

The reaction with the metals that are brightened causes a reduction of the chromic acid to trivalent chromium. As the solution is used, the trivalent chromium content increases, and finally reaches a concentration which causes a brown coating, which is insoluble in water, to form on the metal. In our preferred solution containing 150 grams per liter chromic acid (CrOa) and 3.75 grams per liter sulphate radical (S04), the maximum allowable concentration of trivalent chromium is about 10 grams per liter, corresponding to the solution of about 40 grams per liter of cadmium. For continuous operation to brighten without a subsequent acid or alkali rinse, it is therefore desirable to re-oxidize the trivalent chromium to hexavalent. 'I'his is successfully done by means of the porous cup method used for a similar purpose in chromium plating solutions. However, the nlm formed by an excess of trivalent chromium can also be removed in the dips previously described. The film formed by an excess concentration of trivalent chromium is of the same nature as that formed when the ratio of grams per liter CrOa to grams per liter S04 or grams per liter NO3 or to grams per liter Cl falls below' the critical ratios abovementioned.

The chromic acid bright dip can also be used for brightening of plate which has been in use for a period of time and has become tarnished, stained and dirtied. The dilute nitric acid bright dip which has been proposed heretofore, on such a plate causes dirty dark spots on the immersion of even partially dry or dirty surfaces.

This dip of chromic acid and an active acid radical. as above described, not only brightens metals electronegative to iron and belonging to` the second sub-group of the second group of the periodic system, but also prevents tarnishing of these metals when plated from a cyanide solution as described and claimed in copending application Serial No. 627,423.

Although the method has been particularly described in reference to brightening the surfaces of electrodeposited metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system, it is specifically understood that the above set forth method and dip for brightening the surfaces of these metals is applicable to the surfaces of these metals whatever their genesis may be, that is, whether electrodeposited, sprayed, rolled, cast or otherwise formed.

We claim:

1. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a, solution of chromic acid and a catalystv in the form of a sulphate radical and maintaining the ratio in grams per liter of chromic acid to sulphate radical below a critical ratio of about 20, or maintaining an equivalent ratio in grams per liter of chromic acid to an acid radical or radicals equivalent to the sulphate radical below an equivalent critical ratio to permit formation of a discolorlng film on said surface, then subjecting the said metal and nlm to the action of a solution of any acid having a pH value of 2.68 or less.

2. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and a catalyst in the form of a sulphate radical and maintaining the ratio in grams per liter of chromic acid to-sulphate radical below a critical ratio of about 20, or maintaining an equivalent ratio in grams per liter of chromic acid to an acid radical or radicals equivalent to the sulphate radical below an equivalent critical ratio to permit formation of a discoloring film on said surface, then subjecting the said metal and lm to the action of a solution of any acid having a pH value of 2.68 or less.

3. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a dip of chromic acid and a catalyst in the form of an active inorganic acid radical from a group consisting of the sulphate, chloride and nitrate radicals wherein the chromic acid content of the dip ranges from grams per liter to the saturation point, and the ratio of grams per liter of chromic acid to grams per liter of sulphate radical falls below a critical ratio of 20 or below an equivalent critical ratio of about 12 when the nitrate radical is used and an equivalent critical ratio of about 10 when the chloride radical is used, to permit the formation of a discoloring nlm on said surface, then subjecting the said metal and film to the action of a solution of any acid having a pH value of 2.68 or less.

4. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a dip oi chromic acid and a catalyst in the form of an active inorganic acid radical from a group consisting-of the sulphate. chloride and nitrate radicals wherein the chromic acid content of the dip ranges from 25 grams per liter to the saturation point, and the ratio of grams per liter of chromic acid to grams per liter of sulphate radical falls below a critical ratio of or below an equivalent critical ratio of about 12 when the nitrate radical is used and an equivalent critical ratio of about 10 when the chloride radical is used, to permit the formation of a discolorlng lm on said surface, then subjecting the said metal and film to the action of a solution of any acid having a pl-I value of 2.68 or less.

5. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and the catalyst in the form of an acid radical selected from the group of acid radicals consisting of sulphate, chloride and nitrate radicals and in the presence of an excess concentration of trlvalent chromium forming a discoloring lm on the said metal, then subjecting the said metal and film to the action aisasva of a solution oi' any acid having a pH value of 2.68 ox'- less.

6. A. method ot treating metals electronegative to iron and belonging to the second sub-group oi the second group oi' the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and the catalyst in the form of an acid radical selected from the group of acid radicals consisting of sulphate, chloride and nitrate radicals, reacting the metal with the chromic acid to reduce the latter and form an excessive concentration of trlvalent chromium sumcient to cause a discolorlng illm on the said metal, then subjecting the said metal and nlm to the action of a solution of any acid having a pH value of 2.68 or less.

7. A method of treating metals electronegative to iron and belonging to the second sub-group of the second group of the periodic system to produce a high lustre comprising immersing the metal in a solution of chromic acid and the catalyst in the form of an acid radical selected from the group of acid radicals consisting of sulphate, chloride and nitrate radicals, reacting the metal with the chromic acid to reduce the latter and form an excessive concentration of trlvalent chromium sumcient to cause a discoloring iilm on the said metal, then subjecting the said metal and nlm to the action of a solution of any acid having a pH value of 2.68 or less.

GEORGE DUBPERNEL. KARL GUSTAF SODERBERG. 

